X-ray baggage inspection systems are used at airports and other controlled access facilities, for example to look for contraband in checked or hand luggage and other packages. At an airport, the bags typically move on a conveyor belt through an x-ray beam and an operator looks at the resulting image and other information to spot contraband. The assignee of this invention Vivid Technologies, Inc. ("Vivid") markets dual-energy x-ray baggage inspection systems that use differences in the response of materials to different x-ray energy ranges and further use various signal processing techniques to identify individual objects in a bag and to derive information regarding the effective atomic number and other information for such individual objects and thus assess the likely presence of contraband materials. Some systems measure only transmitted x-rays while others measure scattered radiation as well. Vivid offers such systems under trade names such as VIS or VDS, and is the assignee of U.S. Pat. No. 5,319,547 (Krug, et al.), U.S. Pat. No. 5,490,218 (Krug, et al.), U.S. Pat. No. 5,600,700 (Krug, et al.) and U.S. Pat. No. 5,642,393 (Krug, et al.), which are hereby incorporated by reference into this disclosure as though fully set forth herein. Other modalities also have been proposed or used in connection with seeking to detect contraband in baggage and other packages, such as single-energy computerized tomography (CT) to find the density of materials, dual-energy CT to find the effective atomic number as well, vapor analysis seeking to detect traces of chemicals associated with contraband, thermal neutron technology, and nuclear quadrupole resonance (NQR) technology. See, for example, U.S. Pat. No. 4,686,695 (Macovski), U.S. Pat. No. 5,647,018 (Benjamin), U.S. Pat. No. 5,182,764 (Peschman, et al.), U.S. Pat. No. 5,692,028 (Geus, et al.), U.S. Pat. No. 5,379,334 (Zimmer, et al.), U.S. Pat. No. 4,366,382 (Kotowski), U.S. Pat. No. 5,692,029 (Husseiny, et al.), U.S. Pat. No. 5,600,303 (Husseiny, et al.), and U.S. Pat. No. 5,442,672 (Bjorkholm, et al.), U.S. Pat. No. 5,608,321 (Garroway, et al.), U.S. Pat. No. 5,457,385 (Sidney, et al.), U.S. Pat. No. 5,491,414 (Smith, et al.), and U.S. Pat. No. 5,583,437 (Smith, et al.). All of the patents identified above are hereby incorporated by reference into this disclosure. In addition to the equipment sold by Vivid Technologies, Inc., equipment for detection of contraband has been offered by companies such as, e.g., Heimann Systems of Frankfurt, Germany, Rapiscan Security Systems of Long Beach, California, InVision Technologies, Inc. of Foster City, Calif., and EG&G Astrophysics, Inc. of Long Beach, Calif.
It is believed that existing baggage inspection systems meet some but not all of the inspection needs, such as a high throughput rate for efficient passenger baggage handling, high detection rate to increase the likelihood of finding contraband, and a low false alarm rate to reduce the need for additional time and resources in further checking of suspicious bags. In the case of checked baggage, some airports use several levels of checked luggage inspection. For example, a level 1 station uses an automated x-ray system which inspects luggage at a high rate such as over 1000 bags per hour but has relatively high false alarm rate such as from 10% to 50%. A bag that cannot be cleared at a level 1 station is diverted to a level 2 station, where typically an operator examines an x-ray image of the bag and looks for shapes that may suggest contraband such as detonation devices, wires, weapons or other suspicious objects. A bag that is not cleared at level 2 typically goes to a level 3 station where it is subjected to a more stringent examination. While such equipment and procedures can give satisfactory performance, a need still remains to speed up inspection while maintaining or increasing the detection rate and maintaining or decreasing the false alarm rate. This disclosure describes a system and a method directed toward meeting that need.